The mitotic spindle in most eukaryotes, including humans, is organized by centrosomes. These organelles nucleate the formation of microtubules that capture chromosomes and segregate the chromosomes into daughter cells at mitosis.
Most cells have a single centrosome throughout most of the cell cycle. Before mitosis, a new centrosome is formed adjacent to the first centrosome. The two centrosomes then form the poles of the mitotic spindle. Upon completion of mitosis and cytokinesis, cells, which transiently have two centrosomes, resolve into two cells, with each containing a single centrosome. Normal cells are rarely observed with more than two centrosomes.
Recently, centrosomal proteins such as pericentrin and .gamma.-tubulin have been identified. Anti-sera raised against these proteins can be used to identify centrosomes and centrosomal components in mammalian cells.